This avoids inconsistencies between the LITEST_ enum value and the
shortname but also makes it easier to grep for any test cases that use
the same define. At the cost of the test names not looking very nice
anymore but oh well.
Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1157>
Where a more generic match assigns a palm threshold to a device, allow
unsetting this by assigning a threshold of zero.
And remove the bug log for palm size threshold of 0 for the same reason.
